Method for rotary bi-axle type friction welding

ABSTRACT

This disclosure relates to a method and apparatus for friction welding wherein an improved friction welding of a first and a second workpieces are carried out in a manner such that welded and plasticized workpieces are rapidly synchronized and also a larger thrust is provided at only the end stage of the welding so that change of metallic structure of welded workpieces due to cooling which tends to cause cracks in the welded portion is prevented. In order to carry out this improved friction welding, the apparatus for friction welding includes a second driving means for driving the second workpiece, detecting means for detecting the sum of the lengths of welded workpieces capable of providing a signal for interrupting the drive of the first workpiece and hydraulic systems capable of adjusting the thrust according to a signal from the detecting means.

United States Patent [1 1 Takagi et a].

[11] 3,819,339 June 25, 1974 METHOD FOR ROTARY BI-AXLE TYPE FRICTION WELDING [75] Inventors: Nobuo Takagi, Kariya; Takeshi Kubo, Oobu; Takashi Takiguchi, Kariya, all of Japan [73] Assignee: Kabushiki KaishaToyoda Jidoshokki Seisakusho, Aichi-ken,

Japan [22] Filed: Dec. 28, 1971 [211 App]. No.: 212,910

[301 Foreign AppIicationPriority Data Dec. 30, 1970 Japan 45-123068 [52] US. Cl 29/4703, 156/73, 228/2 [51] Int. Cl. B23k 27/00 [58] Field of Search 228/2; 29/4703; 156/73 [56] References Cited UNITED STATES PATENTS 3,538,592 11/1970 Padilla 228/2 X 3,549,076. 12/1970 Mills 228/2 3,595,462 7/1971 'Hirayama 29/4703 X 3,623,213 11/1971 Sciaky et a1. 228/2 3,678,566 7/1972 Ellis et al. 228/2 X 3,712,528 l/l973 Takagi et al. 228/2 3,753,820 8/1973 lshikawa et al 156/73 Primary Examiner-J. Spencer Overholser Assistant Examiner-Robert J. Craig [57] ABSTRACT This disclosure relates to a method and apparatus for friction welding wherein an improved friction welding of a first and a second workpieces are carried out in a manner such that welded and plasticized workpieces arev rapidly synchronized and also a larger thrust is provided at only the end stage of the welding so that change of metallic structure of welded workpieces due to cooling which tends to cause cracks in the welded portion is prevented. In order to carry out this improved friction welding, the apparatus for friction welding includes a second driving means for driving the second workpiece, detecting means for detecting the sum of the lengths of welded workpieces capable of providing a signal for interrupting the drive of the first workpiece and hydraulic systems capable of adjusting the thrust according to a signal from the detecting means.

6 Claims, 12 Drawing Figures NUMBER OF ROTATION s AMOUNTOF UPSETTING PATENTEIJJUN25I974 3,819,339

sum 2 or 5 NUMBER OF ROTATION NUMBER OF ROTATION AMOUNT OF UPSETTING Fig. 5

NUMBER OF ROTATION AMOUNT OF UPSETTI NG PATENTEDJUNZEIW 3.819.339

SHEET u 0P5 NUMBER OF ROTATION TIME WELDING The present invention relates to an improvement in the method for rotary bi-axle type friction welding and an apparatus to carry out the improved method.

There is a conventional method and an apparatus for bi-axle type friction welding in which the first workpiece rotating at high-speed and the second rotatable workpiece co-axially facing the first workpiece are brought into friction contact under the action of an axial thrust force so as to be welded to each other while the extrusion of a welding fin producedby the action of frictional heat generated at the contact surfaces of both workpieces being welded and the abovementioned axial thrust force, is removed continuously by using the rotational motion of the workpieces being welded and further, the generation of heat required forfriction welding terminates when the relative rotational speed of both workpieces becomes zero.

However, in the above-mentioned conventional rotary type bi-axle friction welding method. the following drawbacks are encountered.

1. Generally, as the rotation of the second workpiece is synchronized with the high speed rotationof the first workpiece, the driven spindle holding the second workpiece retains large kinetic energy even after completion.

of inertia of the flywheel; stepless or continuous adjustment can not be performed and. is accompanied by ex-, treme difficulties in the adjusting operation. 3. Because of the presence of cutting resistance in removing the welding fm while the frictional heat is being generated, and of variation of the idling torque loaded to the driven spindle, the friction welding-energy varies and thus the overall length of the welded workpieces tends to be irregular on completion of the welding.

4. As it takes a long time to reach synchronization of rotations of both workpieces from-the start of the operation change of metallic structure is caused bythe cooling of both workpieces during their long relative motion and thus poor welding tends to result, which weakens the junction at the welded, portion of both workpieces. This defect is prominent in workpieces of small diameter and therefore, of small thermal capacity and also, when welding some particular materials such as heat resisting steel having extremely bad weldability.

The principal object of the present invention is to attain rapid synchronization between the rotations of both workpieces in the relatively low rotational speed range by stopping the application of driving power to sum of the lengths of the workpieces which have been frictionally welded.

v thrust at the end periodof the generation of frictional heat so that additional heat is provided so as to boost the welding heat immediately before reaching synchronization between the first and the second workpieces and also i so that the synchronization between both workpieces can be attained quicker.

The method and apparatus for rotary bi-axle type friction welding in accordance with the present invention will be apparent from the ensuing description of typical embodiments of the present invention with reference to the accompanying drawings wherein:

FIG. 1 is a schematic front view including a partially sectional view of an embodiment of a friction welding apparatus according to the present invention;

FIG. 2 is an illustrative partial view showing means for detecting the sum of the lengths of the two workpieces in the apparatus of FIG. 1;

FIG. 3.to FIG. 5 are graphical diagrams showing the relationship between the number of revolutions of the workpieces and upsetting amount of workpieces in connection with illustration in operation of the friction welding apparatus of FIG. 1;

FIG. 6 is a schematic front view of another embodiment of a frictional welding apparatus according to the present invention;

FIG. 7 is a schematic'drawing illustrating the hydraulic'circuit applied to the apparatus of FIG. 6;

FlGl' 8 is a graphical diagram showing the relationship between the number of revolutions of workpieces and time in connection with the illustration of the oper ation of the friction welding apparatus of FIG. 6;

FIG. 9 is a schematic drawing of the hydraulic circuit used for the principal control of the thrust in the friction welding apparatus according to the present invention;

the main spindle holding the first workpiece being welded while frictional heat is still beinggenerated.

Another object of the present invention is to obtain workpieces havinga constantlength after completion of friction welding bythe above-mentioned stopping of the application of driving force to the main spindle FIG. 10 is a graphical diagram showing the relationship between welding pressure and time in connection with illustration of action of the hydraulic circuit of FIG. 9;

FIGS. 11 and 12 are photographs illustrating a poorly welded portion and its cross sections.

Firstly, the friction welding apparatus of the present invention shown in FIGS. 1 and 2 will be explained in detail. 7

Referring to FIGS. 1 and 2, a main driving motor 1 is mounted on a suitable portion of bed 2 of the friction welding apparatus. A main platform 3 is fixedly mounted on side (the left hand side in FIG. 1) of bed 2, while at the left hand side of platform 3, the first main spindle 4 is rotatably supported by bearing 5. Be-

tween a driven pulley 6 attached to the left end of the main spindle 4 and-a driving pulley 7. of main driving I side of main platform 3 so as to be coaxial with first based upon the command from the detection of the mainspindle 4 and, a chucking member 12 for holding a first workpiece 11 is fixedly attached to the right hand end of the second main spindle 9.

A clutch means 13 is disposed between the first main spindle 4 and the second main spindle 9.

A braking means 14 is disposed at a portion of main platform 3 so as to be capable of providing a braking action on the second main spindle 9. A subordinate platform 15 is mounted on a base 16 fixed on the other side (the right hand side in FIG. 1) of bed 2 so as to be capable of axially moving back and forth along base 16, and is also coupled with a piston rod 17 of a thrust cylinder (not shown in FIG. 1). A secondary driving motor 19 and a transmission means 20 directly connected to the secondary driving motor 19 are placed on a support plate 18 which extends from the right hand end of the subordinate platform 15 along base 16. A first subordinate spindle 21 is rotatably supported by bearing 22 in the right hand side of subordinate platform 15, and one end of first subordinate spindle 21 is connected to transmission means 20.

A braking means 23 positioned at a suitable portion of subordinate platform 15 is capable of providing braking action to the first subordinate spindle 21. A second subordinate spindle 24 is rotatably supported by a bearing 25 in the left hand side of subordinate platform 15 coaxial with the first and second main spindles 4, 9 and first subordinate spindle 21. The second subordinate spindle 24 couples with the first subordinate spindle 21 at the right end of spindle 24 by way of clutch means 26 disposed therebetween, and to the left hand end of the second subordinate spindle 24, there is attached a chucking means 28 for holding a second workpiece 27 which faces first workpiece 11.

Reference numeral 29 shows a cutting tool used for removing the welding fin of the workpieces to be welded.

Referring now to FIG. 2, a dog member 30 is attached to a portion of subordinate platform 15 so as to cooperate with a limit switch 31, and according to a command signal from this limit switch 31, e.g., the disconnection of clutch means 13 is carried out.

Now, the operation and effects of this embodiment of the present invention will be more apparent from the following description referring to FIG. 1 to FIG. 3.

When main driving motor 1 is activated, the first main spindle 4 is driven by motor 1 by way of driving pulley 7, belt 8, and driven pulley 6 and then the second main spindle 9 is caused to rotate by the clutch means 13, and as a result the first workpiece 11 held by chucking member l2 of second main spindle 9 rotates at a predetermined rotational speed N. When the secondary driving motor 19 is activated, the first subordinate spindle 21 is driven by the motor 19 by way of transmission means 20 at a controlled rotational speed.

The rotation of the first subordinate spindle 21 is then transmitted to a second subordinate spindle 24 by clutch means 26, so that the second workpiece 27 held by chucking member 28 rotates at a rotational speed N Y contact with first workpiece l1, and frictional heat gencrates at the contact surfaces of both workpieces due to the relative motion between both contact surfaces, so that portions at the contact surfaces are brought into a plasticized state, and therefore, the subordinate platform 15 under a constant thrust force gradually advances corresponding to the advance of the welding process in the above-mentioned plastic portions of both workpieces 11 and 27.

When the sum of the lengths of both workpieces 11 and 27 reaches a preselected value, (the value is shown as point A in FIG. 3), detecting means, such as a limit switch, detects the value and then, the clutch means 26 of the subordinate spindle system is disconnected by a command signal from the detecting means so as to release the second subordinate spindle 24 from a constant low speed rotation. The second workpiece 27 now follows the rotational motion of the first workpiece l1 and the rotations of both spindles rapidly synchronize with each other (point B of FIG. 3), so that generation of frictional heat at the contact surfaces of both workpieces 11 and 27 ceases and the friction welding process is completed after application of additional thrust for a 1 minute period.

The welding fin produced at the welded portion of both workpieces is continuously removed by a cutting tool 29.

The area shown by oblique lines in FIG. 3 indicates the welding energy necessary for friction welding of the workpieces 11 and 27 at their contact portions.

This welding energy can be freely adjusted by selectively controlling the disconnecting time of the subordinate spindle system from the secondary driving source which time is determined from the welding conditions such as material and diameter of workpieces.

The above-mentioned controlling of the disconnecting time is carried out by the detection of the upsetting amount of workpieces to be welded.

As explained above, in this embodiment of the present invention, both spindle systems are provided with driving motors, and the second workpiece of the subordinate spindle system which rotates at a predetermined low speed, makes frictional contact with the first workpiece of the main spindle system which rotates at a speed higher than said predetermined low speed of the subordinate spindle system and then, when the sum of the lengths of both workpieces reaches a preselected value, the subordinate spindle system is disconnected from its driving source so that both spindle systems quickly synchronize with each other. As a result of this, as cutting resistance and variation of the idling torque can be decreased during the process of the removal of the welding fin, the variation of the sum of the lengths of both workpieces after friction welding can be reduced to a minimum.

Also, as the moment of inertia of the second subordinate spindle can be made the smallest possible value, the period immediately before the synchronization of both spindle systems wherein large torque is generated, can be shortened and accordingly the maximum values of the torques generated on both spindle systems can be held to low values. As a result of this, junction strength of the welded portion of both workpieces is remarkably increased. Further as adjustment of the energy used for friction welding can be carried out by changing the rotational speed of the subordinate spindle system, the conventional flywheel is unnecessary and easy adjustment is obtainable.

The operation of the above-mentioned friction welding apparatus can also be controlled as follows.

Firstly, both workpieces 11 and 27 rotating at high and low speeds, respectively, are brought into contact with each other so as to generate frictional heat at the contact surfaces of both workpieces and the contact portions of both workpieces l1 and 27 are plasticized by the frictional heat. When the sum of the lengths of both workpieces reaches a predetermined value (point A of FIG. 4) by gradually advancing the subordinate platform 15, dog-leg 30 provided on the subordinate platform comes into contact with limit switch 31 provided onbase 16. When the signal from limit switch 31 actuates clutch means 13 of the main spindle system so as to disconnect the second main spindle 9 from first main spindle 4 in order to release second main spindle 9 from being driven at high rotational speed, the main spindle system containing the first workpiece ll quickly decreases in. rotational speed due to the action of friction force generated at 'the contact surfaces of the workpieces l1 and 27. When themain spindle synboth spindle systems are stopped by means of respective braking means 14 and 23 (at the point C of FIG. 4). However, as described above, the rotational speed at which both workpieces are synchronized, is small, so that the kinetic energy remaining in both spindle systems is extremely small compared with the conventional method and as a result the above-mentioned v stopping operation by both braking means 14 and 23 is easily carried out. w

The above explanation is described referring to FIG. '4 in which, as soon as the sum of the lengths of both workpieces to be welded reaches a predetermined value (at point A), the second main spindle 9 only is released from the. frictional driven motion by the signal from limit switch 31 so as to quickly synchronize both spindle systems (at point B), and thereafter, both spindle systems are stopped by the braking means 14 and 23. However, quicker synchronization of both spindle systems is'obtainable (point E of F IG. 5), if the subordi nate spindle system is released from the driving source by disconnecting the clutch means 26' as commanded by the signal from limit switch 31 as soon as the sum of the lengths of both workpieces 11 and 27 reaches a predetermined value (point D of FIG. 5).

In this case, the kinetic energies of both spindle systems are considerably reduced so that thermal energies generated in braking means 14 and 23 are as small as in'the case of FIG. 4.

The areas shown by oblique lines in FIG. 4 and FIG. 5 indicate welding energies necessary for friction welding in the above-mentioned respective cases.

These welding energiescan be freely adjusted by selectively controlling the disconnecting time of main spindlesystem or the disconnecting times of the main their driving source, which time is determined from the upon the detection of the amount of upsets of the workpieces.

As explained above, these operation methods in the friction welding apparatus of the present invention provide the same effects as the method explained with reference to FIG. 3. Since the main spindle system rotating in a high speed range is released from the driving source when the sum of lengths of workpieces, reaches a predetermined value so that main spindle system and subordinate spindle systems are quickly synchronized with each other in a low rotational speed range, the kinetic energies remaining in both spindle systems after completion of friction welding are quite small. As a result, the braking means for stopping both spindle systems need not be capable of absorbing large quantities of heat and also, can be made small in capacity and overall size.

The friction welding apparatus shown in FIGS. 6 and 7 illustrates another embodiment of the present invention.

In this rotary bi-axle type friction welding apparatus, a particular operation is carried out in such a manner that during the generating process of frictional heat due to the friction contact between the first workpiece of the motor driver rotating main spindle system and the second workpiece of the frictionally driver rotating subordinate spindle system, the subordinate spindle system is continuously partially braked so as to suitably restrain it from increase in rotational speed, and thereafter when the sum of the lengths of both workpieces reaches a predetermined value, the above-mentioned partial braking applied to the subordinate spindle system is released so as to synchronize the rotations of both spindle systems.

As will be understood from FIG. 6, the arrangement and function of the main spindle system is quite similar to the previous embodiment, therefore, the arrangement andfunction of subordinate spindle system only will be principally explained.

Referring to FIGS. 6 and 7, there is mounted on base 16 fixedly positioned on the right hand side of bed 2, a subordinate platform 15 capable of sliding back and I forth and coupled with piston rod 48 of a hydraulic cylinder 47 fixed to a suitable portion on bed 2. Subordinate spindle 24 is rotatably supported by subordinate spindle system and subordinate spindle system from y platform 15 by bearings 25 so as to be coaxial with the first and second main spindle 4 and 9. At the left hand end of subordinate spindle 24, there is fixed a chucking member 28 for holding a second workpiece 27 facing first workpiece 11 so as to face the chucking member 12 of first workpiece 11 and to the right hand end of subordinate spindle 24, a brake disc 53 is fixedly connected. On the above-mentioned subordinate platform 15, there is mounted the main body 54 of the braking device in which hydraulic cylinders 56 are symmetrically arranged while facing each other. In' both hydraulic cylinders 56, there are fitted pistons 57, of which the piston rods 58 have pads 59 attached to the inner ends of piston rods 58 which press against the abovementioned brake disc 53 so as to apply a brake to the subordinate spindle 24.

Dog-leg 40 projecting from the front end surface of the subordinate platform 15 is capable of actuating the limit switch 41 provided at a suitable position on base l6 or bed 2. A cutting tool 29 is provided for removing the welding fin.

Next, the electric and hydraulic circuits of this embodiment shown especially in FIG. 7 will be explained.

Directional control valve 65 connected to hydraulic power source 63 by means of oil line 64 is properly operated by solenoid 65a electrically connected to pressure switch 66 which is switched ON when the hydraulic pressure within cylinder chamber (a) of the above-mentioned hydraulic cylinder 47 reaches a predetermined value, and by solenoid 65b which is electrically connected to limit switch 41.

Oil line 67 connected to directional control valve 65 at one end thereof, runs to piston rod sides within the cylinder 56, while oil line 68 branching into two lines on the way, runs to head sides (b) within the cylinder 56 on the one hand and also runs to relief valve 69 on the other hand so as to maintain the pressure within the head sides (b) at a predetermined value.

The operation and the effect of this embodiment will now be explained in detail.

In the state of continuous rotation of main spindle system driven by the driving motor 1, subordinate platform is advanced (to the left in F IG.'6) by supplying hydraulic fluid to operate piston rod 48 into the cylinder chamber (a) of the hydraulic cylinder 47 and the second workpiece 27 held by chucking member 28 comes into contact with the first workpiece ll of the main spindle system under a preselected thrust.

The friction force generated at the contact of both workpieces I1 and 27 causes the subordinate spindle system including second workpiece 27 to rotate, and at the same time when the hydraulic pressure within cylinder chamber (a) of hydraulic cylinder 47 rapidly rises to a predetermined value, pressure switch 66 is switched *ON and operates solenoid 65a, and thereby directional control valve 65 is switched (to the left in FIG. 7) so as to connect oil line 64 with line 68.

As a result of the above operation, hydraulic fluid is supplied to the head-sides (b) of the hydraulic cylinder 56 and pads 59 attached to piston rods 58 are forced into contact with the surfaces of brake disc 53 so that a brake force is applied to the subordinate spindle system.

As the hydraulic pressure within both head sides (b) of hydraulic cylinder 56 is controlled at a suitable value by means of relief valve 69, the above-mentioned brake force applied to the subordinate spindle system is a partial brake force and therefore, the rotational speed of the subordinate spindle system accelerates more slowly.

Then, as the junction of both workpieces is plasticized by frictional heat, subordinate platform 15 gradually advances so that dog-leg 40 comes in contact with and actuates limit switch 41 when the sum of the lengths of both workpieces reaches a predetermined value.

Therefore, solenoid 65b is actuated to directionally control. valve 65 (to the right in FIG. 7) so asto connect oil line 64 with oil line 67. At that time, solenoid 65a is automatically switched to the OFF position. As a result of the above-mentioned operation, by-

draulic cylinders 56 are supplied with hydraulic fluid at both workpieces now ceases and the friction welding of both workpieces is completed.

In the above-mentioned operation, it is, of course, possible to carry out an additional operation in a similar manner to the previous embodiment in which clutch means 13 is disconnected by the signal from the limit switch 41 so that the synchronized rotational speed of both workpieces is lower and further, the synchronizing occurs earlier.

Also, the arrangement of this embodiment may differ from that of the previous embodiment in the point that the subordinate spindle system of the later embodiment is arranged so as to be given a partial braking force the same as in the conventional system, while subordinate spindle system of the former embodiment is positively driven by a driving source.

However, the concepts of both embodiments do not differ from each other, since the subordinate spindle systems in both embodiments are restrained from the increase of rotational speed which would occur under the frictional driving torque at the contact surfaces of both workpieces to be welded.

Therefore, it will be understood that a similar effect is obtainable in the latter embodiment.

As mentioned in the introductory description, in the conventional rotary type bi-axle friction welding method, the time required for synchronizing of both workpieces after the commencement of the decrease of relative rotational speed between them is very long, and the synchronizing time tends to be substantially prolonged. As a result of this, while the relative rotational speed between both workpieces is decreasing, the junction of both workpieces cools down accompanied by a variation of the metallic structure thereof and as a result unsuitable weldingmaterials such as heat resisting steel is often poorly welding.

Now, the results of laboratory welding of test pieces of the heat resisting steel bar of Japanese Industrial Standard Class G 4302 No. 3 and No. 31 (SUH 3B and SUH 318) which was carried out by the inventors under the friction welding conditions shown in Table I using the conventional friction welding apparatus, are shown in FIG. 11 and FIG. 12 of the accompanying photographs. As is seen fromthe photographs, almost all the test pieces formed cracks at the welding junction.

TABLE I Items Conditions Diameters of test pieces l3 mm The revolutional number of the main, 2400 r.p.m. spindle system Thrust pressure 16 kg/mm Specific energy 5 kg'm/mm ing signal controls the thrust pressure to be applied to both workpieces.

form of a rotary type bi-axle friction welding apparatus.

To the working line (L1) of hydraulic cylinder 71', there are connected in series, a directional control valve 73, a flow-control valve 74, a pressure-reducing valve 75, a directional control valve 76 and a pump 77. Also, a pressure-reducing valve 78 and a check valve 79 are arranged on lines L2 and L3 in parallel with flow-control valve 74 and pressure-reducing valve 75.

Pressure-reducing valve 78 has a set value higher than pressure-reducing valve 75.

A safety valve 80 is provided between directional control valve 76 and pump 77.

Reference numeral 81 shows a'reservoir, and reference symbol (L4) shows a working line provided between hydraulic cylinder 71 and'directional control valve 76 for the purpose of returning the piston rod 72.

In the initial friction welding process, the hydraulic thrust control device having the above-mentioned arrangement is actuated through lines (L1) and (L4) from approximately the initial moment of contact of both workpieces held-by the main spindle system and the subordinate spindle system of the rotary type bi-f axle friction welding apparatus of the present invention.

That is to say, pressure fluid supplied by pump 77 passes into working line L1 by way of the directional control valve 76 according to energization of solenoid (a) of directional control valve 76 and then, it is reduced in pressure by pressure-reducing valve 75 and its rate of flow is reduced by the flow-control valve 74 and thereafter, the fluid is fed to the pressurized side (that is, the right hand side in the drawing) of the hydraulic cylinder 71 via directional control valve 73. As the hydraulic fluid which has passed through flow-control valve 74, is restricted in its flow, the piston rod 72 in the hydraulic cylinder 71 slowly advances and therefore, the subordinate platform to which is coupled piston rod 72 advances also at a slow rate so that the workpiece held in the subordinate platform side comes I smoothly into contact with the workpiece held in the main platform side while positively rotating.

Therefore, the workpiece of the subordinate platworkpieces, the amount of movement of the subordinate platform from the moment of initial contact of both workpieces.

When both workpieces reach the effective weldable state, the solenoid of the directional control valve 73 is energized by the signal from the above-mentioned detector and thereby, the flow of the fluid is switched from line L1 to the high pressure line L2. Thus, the hydraulic pressure applied to hydraulic cylinder 71 in creases so that the thrust acting between both workpieces increases from the value P1 to the value P2.

Also, as line L2 in contrast with line L1 does not restrict the flow of fluid, the quantity of the fluid flowing along the line L2 increases and therefore, piston-rod 72 of the hydraulic cylinder 71 increases its speed of advance, and consequently the increased speed can result in relatively high thrust and relatively high rotational 'relative speed between both workpieces which is required for effective welding.

Of course, pump 77 must have enough power to produce the above-mentioned required pressure and flow During the above-mentioned process, hydraulic fluid on the drain side (that is, the left hand side in the drawing) of the hydraulic cylinder 71 returns to the reservoir 81 by way of line L4 and directional control valve In order to retract the subordinate platform after completion of the friction welding process, the solenoid the pressurized side of hydraulic cylinder 71 enters line L3 by way of line L1 and directional control valve 73 and returns to reservoir 81 by way of check valve 79 and directional control valve 76.

According to the two stage pressure method obtained from the above-mentioned thrust control device, as thermal drop which tends to occur at the junction of workpieces at the end stage of the friction welding, is completely compensated for and also, as the synchronization of both workpieces is rapidly carried out, even a poor weldable material such as heat resisting material iscapable of being welded so that the junction of the workpieces possesses very high strength without forming cracks at the junction.

Further, as the thrust control apparatus according to the present embodiment'is arranged so that not only is the flow of fluid fed to thehydraulic cylinder increased but also the thrust to both workpieces is increased, the friction welding apparatus according to the present invention completely follows the rapid advancement of form follows the rotation of the workpiece of thejmain platform while being kept under a thrust Pl as shown in FIG. 10 so as to increase the rotational speed necessary to reach the effective weldable state accompanied by generation of frictional heat at the contact surfaces.

cal quantities such as the rotational speed of the subordinate spindle system, the amount of upsets of the upsets of workpieces to be welded and therefore,

the drop of thrust which momentarily occurs in the friction welding process, can be eliminated.

Also, for workpieces which do not require removal of the welding fin or for which it is impossible to remove the welding fin during the friction welding process, a welding method where the second workpiece of the subordinate spindle system is completely prevented from rotation by the brake means and is welded to the first workpiece by the above-mentioned two stage pres- 1 1' sure process and the process of immediately stopping the rotation of the main spindle system, may be carried out according to the present invention.

ln the above-mentioned description, the case in which the second thrust P2 is higher than the first thrust Pl isexplained. However, when workpieces having relatively small diameters are welded, the axial thrust acting between the workpieces to be welded and the frictional torque acting at the contact surface are small, and accordingly, the first and the second thrusts P1 and P2 can be equal.

However, in this case, the flow of fluid fed to the hydraulic cylinder must still be increased when considering the rapid advancement of upsets of both workpieces at the end process of friction welding, and according to the above-mentioned thrust control device of the present invention, the increase of fluid flow can easily be carried out while keeping thrust Pl constant during the full process of friction welding.

That is to say, as shown in FIG. 9, by arranging a bypass line L5 and deleting line L2 including the pressure-reducing valve 78 or by mechanically opening the -flow-control valve 74 after the contact of both work speed, the second workpiece coaxially facing said first workpiece on a common axis, producing a signal corresponding to the time when the welding energy at the I contact portions of said workpieces reaches a predeterreleasing step.

mined value by detecting the amount of change of a physical value that changes with an increase of the amount of upsetting of said workpieces, and releasing at least one of said workpieces to rotate freely with the other workpiece in response to said signal so that the rotational speeds of said workpieces approach and rapidly synchronize with each other.

2. A method of friction welding asclaimed in claim 1, wherein said restraining of the rotation of said second workpiece is effected by driving said second workpiece so that it rotates at a speed lower than said predetermined rotational speed of said first workpiece.

3. A method of friction welding as claimed in claim 2, wherein said releasing step is effected by stopping the drive of said first workpiece so that said rotational speed of said first workpiece synchronizes with said lower rotational speed of said second workpiece.

4. A method of friction welding as claimed in claim 1, wherein said releasing step comprises releasing both of said first and second workpieces to rotate freely with each other so that said rotational speeds of said both workpieces synchronize with each other at a rotational speed below said first predetermined rotational speed of said first workpiece but above said given rotational speed of said second workpiece.

5. A method of friction welding as claimed in claim 4 further comprising the step of increasing said axial thrust in response to said signal.

6. A method of friction welding as claimed in claim 1 further comprising a step of increasing said axial thrust in response to said signal at the same time as said 

1. A method of friction welding comprising the steps of driving a first workpiece to rotate at a first predetermined speed, contacting a second rotatable workpiece to the first workpiece under an axial thrust, restraining rotation of the second workpiece from exceeding a given second speed lower than said first predetermined speed, the second workpiece coaxially facing said first workpiece on a common axis, producing a signal corresponding to the time when the welding energy at the contact portions of said workpieces reaches a predetermined value by detecting the amount of change of a physical value that changes with an increase of the amount of upsetting of said workpieces, and releasing at least one of said workpieces to rotate freely with the other workpiece in response to said signal so that the rotational speeds of said workpieces approach and rapidly synchronize with each other.
 2. A method of friction welding as claimed in claim 1, wherein said restraining of the rotation of said second workpiece is effected by driving said second workpiece so that it rotates at a speed lower than said predetermined rotational speed of said first workpiece.
 3. A method of friction welding as claimed in claim 2, wherein said releasing step is effected by stopping the drive of said first workpiece so that said rotational speed of said first workpiece synchronizes with said lower rotational speed of said second workpiece.
 4. A method of friction welding as claimed in claim 1, wherein said releasing step comprises releasing both of said first and second workpieces to rotate freely with each other so that said rotational speeds of said both workpieces synchronize with each other at a rotational speed below said first predetermined rotational speed of said first workpiece but above said given rotational speed of said second workpiece.
 5. A method of friction welding as claimed in claim 4 further comprising the step of increasing said axial thrust in response to said signal.
 6. A method of friction welding as claimed in claim 1 further comprising a step of increasing said axial thrust in response to said signal at the same time as said releasing step. 